A sensor having an EIS structure includes a semiconductor substrate, on which an insulator is arranged, which is supplied with an electrolyte in measurement operation.
ISFETs are established examples of sensors with an EIS structure, wherein, in this case, the insulator forms the ion sensitive gate insulator of a field effect transistor.
In sensors called LAPS (light addressable potentiometric sensors), photoelectrons are produced in the semiconductor material of an EIS structure by means of a modulated light signal, wherein the generation of photoelectrons depends, in turn, on the electrolyte properties. A basic description of LAPS is given by Hafeman et al. in “Light addressable potentiometric sensor for biochemical systems”, Science 240 (1988) 1182-1185.
ISFETs are more established and better examined than other EIS structures. Therefore, in the following description of the problems in the state of the art, reference is essentially made to ISFETs, wherein it is inherent that these problems are correspondingly given for other sensors with an EIS structure.
Ion sensitive field effect transistors (ISFET) are applied for measuring ion concentrations or special substance concentrations in solutions of different compositions and conductivities. ISFETs are applied for the continuous verification of concentrations in environmental monitoring, in industrial process monitoring, in the foods industry and in biochemical/medical technology. Such applications especially depend on highly precise registration of concentration, quick start-up and minimal long time drift of the sensor, plus an acceptable price.
German patent application 10 2009 002060 describes an ISFET and a LAPS distinguished by special media resistance. Reference is made to the detailed discussion of the state of the art there.
Patent application 10 2009 002060 starts with the problem that Ta2O5 ion sensitive layers, which have favorable properties as regards sensitivity and linearity, are especially unstable in the presence of alkaline media with a pH>10, which leads to such media diffusing through the ion sensitive layer; such media can damage or destroy deeper lying layers. Media resistance is achieved according to the teachings of patent application DE 10 2009 002060 by a multilayer construction, in that an essentially crystalline insulation layer is arranged under the ion sensitive layer contacting the media. More exactly, the ion sensitive sensor having an EIS structure disclosed therein includes a semiconductor substrate, on which a layer of a substrate oxide is produced; an adapting or matching layer, which is prepared on the substrate oxide; a chemically stable, intermediate insulator, which is deposited on the adapting or matching layer; and a sensor layer, which comprises tantalum oxide or tantalum oxynitride, which is applied on the intermediate insulator; wherein the intermediate insulator comprises hafnium oxide or zirconium oxide or a mixture of these oxides, and wherein the adapting or matching layer differs from the intermediate insulator and the substrate oxide in its chemical composition and/or in its structure. The adapting or matching layer can comprise, for example, tantalum oxide or tantalum oxynitride.
The ion sensitive layer and the adapting or matching layer of the sensor of DE 10 2009 002060 have an electrical conductivity, which is certainly small but exceeds the conductivity of the intermediate insulator by orders of magnitude. In this respect, the ion sensitive layer and the adapting or matching layer form the electrodes of a capacitor with the intermediate insulator as the dielectric. This can lead, in given cases, to charging and, therewith, potential differences between the ion sensitive layer and the adapting or matching layer, which can effect a variable shifting of the working point of the ion sensitive sensor since charges depend on temperature and type of medium. Depending on the extent of the effect, a shifting of the working point can mean a considerable degradation of the ion sensitive sensor.